The present invention relates in general to optical amplifiers, and in particular to an optical amplifier configuration including shared pumps for use in optical communication systems.
Optical amplifiers have become an essential component in transmission systems and networks to compensate for system losses, particularly in wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) communication systems, wherein a plurality of distinct optical wavelengths or channels are multiplexed and propagated over an optical medium to a plurality of receivers. In a WDM or DWDM system, one of the most efficient and commonly used amplifier devices is the Erbium-doped fiber amplifier (EDFA), which has a gain bandwidth of about 35 nm in the 1.55 xcexcm wavelength region. Unlike semiconductor optical amplifiers, EDFAs do not require conversion of an optical signal into an electronic signal and back. An EDFA operates by passing an optical signal (e.g. a discrete wavelength) through an erbium-doped fiber segment, and xe2x80x9cpumpingxe2x80x9d the segment with light from another laser, thereby strengthening the optical signal and returning it to its original levels without leaving the fiber optic cable for separate electronic processing.
The segment of fiber may be a segment of doped optical fiber in which the dopant is selected from materials that can produce laser action in the fiber segment. Such materials can include rare earth dopants such as erbium, neodynium, praseodymium, ytterbium, or mixtures thereof. Pumping of a doped fiber segment at a specific pump wavelength causes population inversion among the electron energy levels of the dopant, producing optical amplification of the wavelength division multiplexed optical signals.
The amplifier configuration depends on the spectral bandwidth of the signal to be amplified. C-band amplifiers, for example, operate in the range of approximately 1527 nm to approximately 1565 nm, and L-band amplifiers operate in the range of approximately 1565 nm to approximately 1610 nm. An L-band fiber may be approximately five times as long as the C-band amplifier fiber, or may be doped with approximately 5 to 6 times the erbium provided in the C-band fiber. For a C-band amplifier, in view of the short length of the fiber, pumps operating at approximately 980 nm are efficient. For an L-band amplifier, pumps operating at approximately 1480 nm are more efficient, since the use of a 980 nm pump results in only about 37% to 40% inversion.
Typical configurations of EDFAs include two-stage and three-stage amplifiers. A two-stage optical amplifier, well known in the art, can include two segments of erbium-doped fiber spaced by a relatively short length of undoped fiber, and three pumps (e.g. laser diodes). The first doped fiber segment (i.e., the first stage) may be pumped in a forward direction with an appropriate wavelength and at a sufficient intensity to provide high gain but low noise, while the second doped fiber segment (i.e., the second stage) is pumped in both forward and reverse directions to provide high power. Accordingly, such two-stage amplifiers provide gain at high power, but relatively little noise. Likewise, a three-stage optical amplifier can include three doped fiber segments spaced by two relatively short lengths of undoped fiber, and four pumps.
A problem associated with the use of optical amplifiers, however, is the signal loss associated with a failed laser pump, particularly in two-stage and three-stage configurations. Since the signal power is absorbed as it travels through the length of the fiber, eventually it becomes so weak that the gain reduces to zero and the pumped fiber becomes absorbing rather than amplifying. For example, FIG. 1 shows a prior art three-stage optical amplifier configuration with four separate pumps for amplifying an input signal 109. In the first and second stages, a wavelength of light or is provided or xe2x80x9cpumpedxe2x80x9d in a forward direction by the first pump 101 and the second pump 102, at amplifying fiber segments 117 and 118, respectively. In the third stage, the third pump 103 pumps amplifying fiber segment 119 in a forward direction, and the fourth pump 104 pumps fiber segment 119 in a reverse direction, and an amplified output signal 109a is provided.
FIG. 2 illustrates an exemplary gain vs. wavelength characteristic for the three-stage amplifier configuration of FIG. 1 in five different states. In the first state, as represented by curve 201, all four pumps 101-104 are functioning normally. In the second state, as represented by curve 205, the first pump 101 fails, i.e. suffers a reduction or complete loss of output power, or otherwise ceases to function properly. In the third state, as represented by curve 204, the second pump 102 fails. In the fourth state, as represented by curve 202, the third pump 103 fails. In the fifth state, as represented by curve 203, the fourth pump 104 fails.
As can be seen in FIG. 2, with all four pumps operating normally, a gain of about 26 dB may be obtained in the range of about 1572 to 1605 nm. However, if the first pump 101 fails, a loss of approximately 45 dB is experienced. If the second pump 102 fails, a gain of approximately 10-15 dB is experienced over the same range. If the third pump 103 or the fourth pump 104 fails, a gain of about 21-24 dB is experienced. Thus, as can be seen in FIG. 2, in the event of pump failure, signals at shorter wavelengths are absorbed by the fiber to a far greater extent than signals at longer wavelengths. Moreover, in the three-stage amplifier configuration of FIG. 1, it is seen that while significant reduction in amplifier gain occurs if the third 103 or fourth pump 104 fails, failure of either of pumps 101 and 102 can result in a complete loss of the signal.
Accordingly, there is a need in the art for an optical amplifier configuration that provides failsafe capabilities in the event of pump failure to prevent signal loss.
The present invention is organized about the concept of pump sharing. Instead of each pump providing a wavelength of light directly into an amplifying fiber segment, the outputs of the pumps are combined and then divided before being injected into the fiber segments. Thus, by coupling the outputs of two or more pumps and then splitting the coupled output onto two or more output fibers, a failsafe for preventing signal loss is provided, wherein power from at least one of the pumps is provided to each of the output fibers in the event a pump fails.
In particular, an optical amplifier configuration consistent with the present invention includes: a plurality of optical pump sources, each pump source being configured to provide an output pump signal; a coupler configured to combine the pump signals into a combined signal; and a splitter for receiving the combined signal and splitting the combined signal into a plurality of separate signals, at least one of the separate signals being for introduction into at least one associated fiber segment for amplifying an optical signal.
In one exemplary embodiment, a three-stage optical amplifier configuration includes four pumps, three amplifying fiber segments, a coupler for combining a plurality of pump outputs, and a splitter for dividing the coupled output wavelength into separate output fibers, to be pumped into the fiber segments. The outputs from the first and second pumps are combined by the coupler, and the combined output is split by the splitter in half and provided in a forward direction into the first and second amplifying fiber segments. The third pump pumps the third amplifying fiber segment in a forward direction, and the fourth pump pumps the third amplifying fiber segment in a reverse direction. Thus, if either the first or second pump fails, the output from the remaining operating pump is divided in half and is provided in a forward direction into the first and second amplifying fiber segments, each of the output wavelengths being reduced to 50% of its original power.
In another exemplary embodiment, a two-stage optical amplifier configuration includes three pumps, two amplifying fiber segments, a coupler and a splitter. The output from the first pump is provided in a forward direction into the first amplifying fiber segment. The outputs from the second and third pumps are combined and split, and then provided in a forward and in a reverse direction into the second amplifying fiber segment. Thus, if either the second or third pump fails, the output from the remaining pump is divided in half and is provided in a forward and a reverse direction into the second amplifying fiber segment, each of the output wavelengths being reduced to 50% of its original power.
A method of pumping an amplifying optical fiber segment consistent with the invention includes the steps of: combining a plurality of optical pump signals into a combined pump signal; splitting the combined pump signal into separate pump signals; and introducing at least one of the separate pump signals into the fiber segment.